The Collaborative International Dictionary of English v.0.48:

Diffident \Dif"fi*dent\, a. [L. diffidens, -entis, p. pr. of
   diffidere; dif- = dis + fidere to trust; akin to fides faith.
   See Faith, and cf. Defy.]
   1. Wanting confidence in others; distrustful. [Archaic]
      [1913 Webster]

            You were always extremely diffident of their
            success.                              --Melmoth.
      [1913 Webster]

   2. Wanting confidence in one's self; distrustful of one's own
      powers; not self-reliant; timid; modest; bashful;
      characterized by modest reserve.
      [1913 Webster]

            The diffident maidens,
            Folding their hands in prayer.        --Longfellow.

   Syn: Distrustful; suspicious; hesitating; doubtful; modest;
        bashful; lowly; reserved.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diffident
    adj 1: showing modest reserve; "she was diffident when offering
           a comment on the professor's lecture"
    2: lacking self-confidence; "stood in the doorway diffident and
       abashed"; "problems that call for bold not timid responses";
       "a very unsure young man" [syn: diffident, shy, timid,
       unsure] [ant: confident]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "diffident":
   bashful, boggling, cautious, cowardly, coy, demure, demurring,
   deprecative, deprecatory, faltering, fearful, fearing, fearsome,
   goosy, hesitant, hesitating, in fear, jibbing, jumpy, modest,
   mousy, nervous, qualmish, rabbity, reluctant, retiring, scary,
   scrupling, scrupulous, self-deprecating, self-depreciating,
   self-depreciative, self-distrustful, self-doubting, self-effacing,
   shaky, shilly-shallying, shivery, shrinking, shy, skittery,
   skittish, squeamish, startlish, sticking, stickling, straining,
   tentative, timid, timorous, trembling, tremulous, trepidant,
   trigger-happy, unassertive, unassured, unselfconfident,
   unselfreliant